Workpart chuck positioning mechanism with independent shoes

ABSTRACT

A workpart chuck centering mechanism moves towards the axis of rotation of the chuck as the chuck rotates in an eccentric non-centered position. The centering mechanism includes two independent shoes. Each shoe is driven by a servo-motor and encoder. The two shoes function in a cooperative manner to center the workpart. A sensing device monitors any eccentric movement of the workpart and controls the positioning of the shoes. The two shoes and the sensing device are mounted on a base which can pivot about the chuck axis. The directions of movement of the respective shoes are displaced 90° relative to one another and intersect at the chuck axis.

BACKGROUND

Machine tools such as internal and external grinding machines are knownto utilize workpart centering mechanisms for positioning a workpartrelative to a workpart chuck that rotates the workpart for grindingoperations. In production type of machining operations, magneticfaceplate chucking is less frequently used since time is lost andskilled operator attention is required for centering each workpiece onthe faceplate. A dial indicator and skillfully dealt hammer blows to theworkpiece are required to center the workpiece. Of course, the objectfor the operator is to center the workpiece on the faceplate with thegeometrical axis of the workpiece substantially coaxial with therotational axis of the faceplate that is attached on a rotatablespindle.

Automatic centering control mechanisms using a single drive shoe tocenter the workpart have been developed. However, such systems cannot beemployed in traditional centerless tooling procedures in which theworkpart axis is intentionally offset from the spindle axis for thegrinding operation. A need exists for a workpart chuck centeringmechanism that can be adapted for centerless tooling procedures yetprovides a system employing computer numerically controlled capabilitiesfor automated centering.

SUMMARY OF THE INVENTION

The invention relates a workpart positioning mechanism with a first shoeelement for moving the workpart in a first direction and a second shoeelement that can move the workpart in a second direction. Each shoeelement is fitted with a shoe that can be used to control the positionof a workpart either during a centering procedure or during a centerlessgrinding operation. The system can be adapted for either internal orexternal grinding operations.

The degree of workpart eccentricity or offset from the desired locationis monitored by a sensing device and processed by an automatic controlsystem which controls shoe element movement as well as other grindingmachine components. When used for centering a workpart for grinding, oneshoe (the "positioning shoe") is used to move the workpart towards thecenter. The second shoe (the "following shoe") is maintained a fewthousandths of an inch from the workpart during centering. If theworkpart is moved just beyond the center, the workpart will snap into aposition in contact with both shoes which are then retracted to restartthe centering procedure. By using two centering shoe elements, theworkpart can be repositioned quickly and efficiently. Each shoe can beused as either the positioning shoe or the following shoe depending uponthe direction of rotation of the spindle. Moreover, the two separateshoe elements can be used as stop devices to prevent the workpart frommoving during the grinding operation.

In a preferred embodiment, the first shoe element is moved in adirection that is perpendicular to the direction of movement of thesecond shoe element The first shoe element direction and the second shoeelement direction define axes that can be positioned to intersect at thecenter of the chuck. For applications thus, the workpart axis can beprecisely positioned at the chuck axis. The workpart is positioned onthe chuck by means of either a magnetic or mechanical coupler. The firstand second shoe elements and the position detector are positioned on asupport which pivots about the center of the chuck.

The speed and precision of the two shoe centering mechanism facilitatesthe rapid changeover of tooling workparts for production machineswithout operator assistance. Note further, for an alternative use of thetwo independent shoe system, in which the shoe elements are secured tohold the workpart for a standard centerless grinding operation. In thisembodiment the shoes position the workpart axis at a desired offset fromthe spindle axis. The force imparted to the workpart by the rotatingchuck and magnetic faceplate is directed between the first and secondshoe elements. Thus, both shoes are used to maintain the workpart axisat the desired offset during the grinding operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following detailed description ofthe preferred embodiments of the invention, as illustrated in theaccompanying drawings in which like reference characters refer to thesame parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 illustrates a front view of the preferred embodiment of theinvention.

FIG. 2 illustrates a horizontal view of the preferred embodiment of theinvention.

FIGS. 3A-3C illustrate the centering process.

FIG. 4 illustrates schematically the control system employed forpositioning of workparts.

FIG. 5 illustrates an alternative embodiment employing the two shoes forcenterless grinding.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 illustrate a magnetic workpart chuck 10 for a grindingmachine that can be adopted for internal or external grinding. Notehowever, that the invention is not limited to grinding machines or tomagnetic chucks but can be adopted for use in other machiningoperations.

As is known in applications requiring the centering of a workpart forgrinding, an annular workpart W is held on magnetic chuck faceplate 10with the rotational axis of the workpart and faceplate 10 substantiallycoaxial or aligned. The chuck faceplate is attached on a rotatablespindle and the spindle is rotated by an electric motor or otherconventional and known means. The spindle and motor can include separatecomponents drivingly connected for example by a drive belt, or pulleys,gear train, etc., or the spindle and motor can be integrated to providea motorized spindle. For example, U.S. Pat. No. 4,790,545, incorporatedby reference herein, illustrates centering mechanisms employed in agrinding machine.

Suitable faceplates for use in the invention are available commercially.Known mechanical workpart clamps or chucks can be used in the inventionin lieu of the magnetic faceplace; e.g., roll clamping and air clampingwhere the workpart is held by rollers or air pressure against afaceplate on a spindle can be used in lieu of the magnetic faceplate 10to be described below.

Spindle within housing 22 and electric motor 24 are typically mounted onthe base 26 of the grinding machine or a slide (not shown) movable onthe base.

A grinding wheel 30 is moved into the workpart bore for internalgrinding and rotated and reciprocated against the inner annular surfaceS to grind same. The grinding wheel is radially fed into the annularsurface at a desired feed rate until the final ground dimension isachieved. The mechanism for moving the grinding wheel is well known inthe art e.g., U.S. Pat. No. 4,653,235 issued Mar. 31, 1987, and forms nopart of the present invention.

As is known, during grinding, the workpart W is centered on chuckfaceplate 10 with its rotational (geometrical) axis substantial coaxialwith the rotational axis R of the faceplate and spindle.

Typically, successive workparts are ground one after another until thedesired number have been ground. A workpart loader/unloader is providedto place an unground workpart faceplate 10 after the previously groundworkpart is removed therefrom. Loader/unloader 54 is illustratedschematically (see FIG. 4) and includes a pivotal arm having workpartgrippers. The pivotal arm is pivoted from a source of unground workpartsto carry an unground workpart to a position in front of the faceplate10. When the pivotal arm is in front of the faceplate, the pivotal armis axially slidable in a direction toward the faceplate 10 to deliver aworkpart W to the faceplate 10 and is slidable axially away from thefaceplate and then pivoted to allow grinding.

After grinding of the workpart, the pivotal arm is pivoted and thenmoved axially toward the faceplate to grip the ground workpart andaxially away to remove the ground workpart from the faceplate 10. Thearm is then pivoted to discharge the ground workpart and to pick up anunground workpart for placement on faceplate 20. The sequence ofmovements of the pivotal and axially movable arm is repeated for eachworkpart. Such workpart loaders/unloaders are known in the art; e.g.,available on Bryant 2209D-II internal grinder available from BryantGrinder Corporation, Springfield, Vt. Workpart loaders/unloaders ofother known types can be used in the invention.

Regardless of the workpart loader employed, the loader does not placethe workpart geometrical axis and chuck axis substantially coaxial.Instead, the workpart typically is placed on the faceplate with acharacteristic offset of the workpart axis G from the chuck axis R (FIG.3A) such that rotation of the chuck causes the workpart axis to travelin an eccentric path.

The same off-center positioning of the workpart will be experienced inthe event the workpart is loaded onto the faceplate 10 manually by themachine operator.

FIGS. 1 and 2 illustrate the two shoe element centering mechanisms ofthe invention. Sensor or detector 12 provides a continuous measure ofthe amount of offset of the geometrical center of the workpart 18 fromthe chuck axis. An offset or eccentricity signal from sensor 12 isdelivered to an automatic control device which creates an output drivesignal for each shoe element in accordance with this offset as well as ameasure of the angular position of the chuck (as determined by atachometer referenced to a given position of the chuck workface).

By using a first shoe 14 and a second shoe 16 moving in differentdirections, the workpart can be repositioned to align its geometriccenter with the chuck axis in a rapid and efficient fashion. The twoindependent shoes each are driven by a servomotor and an encoder, orother prime mover, through an anti-backlash worm gear reduction. Forexample, motor 24 drives the worm gear of the first shoe 14 and motor 28drives the worm gear of the second shoe. The internal diameter of a wormgear drives a threaded plunger 34 or element on which shoe 14 ismounted. The plunger rides on bushings and is secured by hydraulicelements 21. The two shoes track each other and function, as describedin connection with FIGS. 3A-3C below, to center the workpart. The shoesact in respective directions which are perpendicular to one another andpreferably intersect at the chuck axis. Servo motors 24 and 28 supplythe automatic controller with data concerning the position therespective shoe drive shafts 32 and 34.

The sensor 12 and first and second shoes are mounted on a shoe support36 or holder that is rotatably mounted to pivot around the chuck axisrelative to the base 26. The support 36 is rigidly bearing mounted tothe workhead base 26 and through a worm gear arrangement and can berotated about the chuck axis to a programmed position that facilitateseither outer diameter on inner diameter grinding. The support 36 isphysically locked in position during grinding. When the support 36 ispivoted, it is hydraulically released. A separate servo motor andencoder drive the movement of the holder 36 relative to base 26.

The automatic control device processes input information concerningeccentricity, angular position of the chuck, linear position of shoeshafts, and angular position of the shoe base to derive a drive signalfor each shoe shaft. A preferred embodiment of the control system isshown schematically in FIG. 4. As a result, the workpart can beprecisely centered on the chuck axis. As a result, the shoes need notcontact the workpart during grinding to contain the workpart. Thus,eccentric offsets of the workpart and the resultant wear on shoetips andworkparts can be eliminated. If the workpart overruns the center of thechuck, the shoes will retract and begin the centering process again.When centering is finally achieved, the shoes are retracted to clear theworkpart.

The operator can program the main computer numerical control (CNC)system 52 to conduct various types of grinding operations. Morespecifically, the computer or programmable data processing system 52controls the operation of the loader/unloader 54, the main spindleoperation including driver 51 and dresser (not shown), and controller56. The controller 56 receives position feedback information from threeservomotors 60, 62, 64, each of which is fitted with a rotary encoder.Servomotor 60 serves to control the movement of the first shoe element14, 34, servomotor 62 serves to control the movement of the second shoeelement 16, 32, and servomotor 64 serves to control the movement of theshoe holder or support 36.

The servoamplifier 58 receives velocity feedback information from eachof the servomotors 60, 62 and 64 and serves with controller 56 toprovide the necessary voltage to the servomotors to accurately controlthe movement of the shoe elements and the holder 36.

The detector 12 includes linear encoder 66 to provide position feedbackinformation regarding the position of the workpart to the controller 56.

The detector 12 can be a long stroke device that provides adequateresolution and is mounted on the shoe holder 36. Alternatively thedetector 12 can be a short stroke device, such as an LVDT, that ismounted on either side of the following shoe element.

The control device can employ automated processing techniques. Forexample, Gile in U.S. Pat. No. 4,926,337 that is incorporated herein byreference, describes a computer microprocessor which utilizes a look-uptable to process chuck angular position and eccentricity information todrive a hammer to reposition a workpart. A similar microprocessor can beused to drive the two shoes of the preferred embodiment.

Shoe inserts 14, 16 take the form of plates permanently or, optionally,releasably attached on the holders 34, 32. The shoe inserts define apocket adapted to receive the off-center workpart on faceplate 10 aswill be explained.

Shoe inserts 14, 16 preferably are positioned relative to the rotationalaxis of the faceplate and spindle to have only shoe insert 14 initiallyengage the workpart and to move the workpart of FIG. 3B toward therotational axis R of the faceplate (see arrow A of FIG. 3B).

Such engagement of shoe insert 14 with the workpart is shown in FIG. 3B.It is apparent that the workpart initially engages only shoe insert 14and not shoe insert 16. This is accomplished by locating the shoeinserts so as to make the imaginary line L which bisects the includedangle formed by the contact surfaces 14, 16 of the inserts (hereafterreferred to as bisector) offset a distance X laterally of the rotationalaxis R. The bisector is offset in a direction opposite to the directionof rotation (arrow C in FIG. 3A), of the faceplate for reasons to beexplained. The statement that the bisector is offset in a directionopposite of the direction of rotation is intended to mean that a pointon the rotating chuck faceplate approaching the centering member 34 willcross the bisector (line L) before it crosses a line that extendsthrough the center of the rotating chuck faceplate and that is parallelto the bisector. Distance X typically is about 10 mils (0.010 inch).

In typical operation of the magnetic workpart chuck, machine control 52sets the magnetic intensity of faceplate to a first predetermined leveland commands the loader/unloader 54 to place a workpart on the faceplateas shown in FIG. 3A. Once the workpart is loaded, the magnetic intensityis changed to a second predetermined level higher than the first level.

The faceplate can be rotated during loading or is rotated immediatelyfollowing loading of the workpart thereon. Since the workpart is loadedoffcenter on the faceplate, the control 56 directs the advance of shoeinserts 14, 16 on holder 36 from an initial starting position toward thechuck rotational axis R. The holder 36 and shoe inserts thereon isadvanced at a first predetermined speed as the faceplate rotates theoff-center workpart with its workpart axis traveling along a eccentricpath. The eccentric path of movement of the workpart changes as it ismoved toward the chuck axis. As the shoe inserts 14, 16 are advanced byservomotors 60, 62, the detector 12 contacts the eccentrically rotatingworkpart. However, as shoe insert 14 continues to move the workpartcloser to the rotational axis R of the faceplate in the direction ofsmall arrow A in FIG. 3B contact between detector 12 and the offcenterworkpart eventually becomes continuous and the amplitude of thecyclically varying signal (approximate sine wave) from the detector 12becomes proportional to the extent or magnitude of eccentric movement ofthe workpart.

When the amplitude reaches a first predetermined level or value, thespeed of advance of the workpart engaged by shoe inserts 14, 16 isreduced optionally to a second lower level or value. When the amplitudeof the signal from detector 12 reaches a second predetermined valuelower than the first value and indicative of the workpart rotating aboutits own axis; i.e., with its axis substantially coaxial with rotationalaxis R, the control directs the servomotors 60, 62 to stop andoptionally to reverse direction to retract shoe inserts 14, 16 away fromthe now centered workpart a selected small amount. This optionalretraction will prevent needless rubbing/wear between the shoe insertsand workpart and yet will position the shoe inserts sufficiently closeto the workpiece to prevent excessive movement of the centered workpartin the event of accidental application of grinding forces exceeding themagnetic and friction holding force between the faceplate and theworkpart.

Moving the workpart toward rotational axis R with the bisector of shoeinserts 14, 16 offset (distance X) has been found to be important incase the workpart is advanced slightly too far past the chuck axis R.The inventors found that with the offset present, the workpart will snapfrom the position of FIG. 3B to the position of FIG. 3C in full contactwith shoes 14 and 16.

In a centerless grinding operation commonly used in grinding cylindricalparts for bearing applications, work support shoes 72, 74 and areadjustably mounted on shoe holder 86 as shown in FIG. 5. Shoe holder 86is, in turn, mounted on the workhead base, the holder 86 position beingcontrolled by servomotor as previously described.

The second shoe 74 is adjustable angularly and is normally set 13 to 15degrees below the horizontal. The first shoe 72 is adjustablehorizontally and is set as far toward the wheelslide as possible withoutmaking contact with the grinding wheel when 90 it is in finish grindposition.

Both shoes are adjustable in respect to their distances from the centerR of the driver in order to provide for different workpiece sizes andfor regrinding. Methods of obtaining shoe locations differ but theultimate settings are much the same in each case. The center G ofworkpiece W is offset from the center R by dimensions 94 and 96. Thisoffset can be adjusted by the operator at 50 if desired. These settingsare such that a finished workpiece, when resting on shoes 72, 74 ispreferably 0.010" below center R of driver at 94 and 0.010" towardgrinding wheel from center R of driver as indicated at 96. Greateroff-set is required if grind stock exceeds 0.020" on diameter.

The usual procedure is to set up the shoes to predetermined positions ona tool room grinding fixture, then grind the shoes to fit a prescribedpart. The shoes are then transferred to the grinder. Any furtheradjustment in shoe location is made by altering the settings of theadjusting screws 84 and repositioning the shoe holder 86.

Once shoes 72 and 74 have been satisfactorily positioned by this methodit should not be necessary to reposition adjusting screws 84 when shoes72, and 74 are later reground to the same prescription. The shoes 72 and74 are mounted in elements 76 and 78 with hold down screws 80 and 82.

The system can also be adapted for centerless internal grindingoperations.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

We claim:
 1. A method for centering a workpart on a rotating chuck comprising the steps of:positioning the workpart on the chuck; rotating the chuck about a chuck axis such that a geometrical axis of the workpart is offset from the chuck axis; measuring the degree of workpart geometrical axis offset from the chuck axis of rotation; positioning a first shoe element in contact with the workpart with a first driver and positioning a second shoe element in proximity to the workpart with a second driver; and moving the first shoe element with the first driver and the second shoe element with the second driver to move the workpart axis toward the chuck axis as the chuck and workpart rotate.
 2. The method of claim 1 wherein the first element moves in a direction perpendicular to a direction of movement of the second element.
 3. The method of claim 2 wherein the first direction and the second direction intersect at the center of the chuck.
 4. The method of claim 1 wherein the first and second drivers comprise first and second servomotors.
 5. The method of claim 1 wherein the workpart is positioned on the chuck with a magnetic holder.
 6. The method of claim 1 wherein the workpart is positioned on the chuck with a mechanical holder.
 7. A workpart chuck apparatus for centering a workpart on a rotating chuck comprising:a rotatable chuck having an axis of rotation; and a coupler for holding the workpart to the chuck; a detector to measure the offset of a workpart geometrical axis from the chuck axis of rotation; a first shoe element positionable to guide movement of a workpart axis in a first direction; a second shoe element positionable to guide movement of the workpart axis in a second direction; and a control system electrically connected to the first driver and the second driver to independently control positioning of the first shoe element and the second shoe element relative to the workpart.
 8. The workpart chuck apparatus of claim 7 wherein the first direction is positioned perpendicular to the second direction.
 9. The workpart chuck apparatus of claim 7 wherein the first direction and the second direction intersect at the center of the chuck.
 10. The workpart chuck apparatus of claim 7 wherein the detector generates an electrical signal correlated with the measured offset that is delivered to the control system.
 11. The workpart chuck apparatus of claim 7 further comprising a support assembly on which the first element and second element are mounted.
 12. The workpart chuck apparatus of claim 11 wherein the support assembly is rotatably mounted such that the support assembly can rotate about the chuck axis of rotation.
 13. The workpart chuck apparatus of claim 7 wherein said coupler further comprises a magnetic holder.
 14. The workpart chuck apparatus of claim 7 wherein the coupler further comprises a mechanical holder.
 15. The workpart chuck apparatus of claim 7 wherein the control system further comprises a first servomotor to control the position of the first element and second servomotor to control position of the second element.
 16. The workpart chuck apparatus of claim 7 wherein the detector comprises a linear encoder.
 17. The workpart chuck apparatus of claim 11 further comprising a servomotor to control the position of the support assembly.
 18. The workpart chuck apparatus of claim 7 wherein the first and second elements each comprise a worm gear to control linear movement of each element relative to the chuck axis.
 19. A workpart chuck apparatus for centering a workpart on a rotating chuck of a grinding machine comprising:a rotatable chuck having an axis of rotation; a coupler for holding the workpart on the chuck; a detector to measure an offset of a workpart geometrical axis from the chuck axis of rotation and generate a feedback signal; a first shoe element positionable to guide movement of a workpart axis in a first direction, the first shoe element being mechanically driven by a first servomotor; a second shoe element positionable to guide movement of the workpart axis in a second direction, the second shoe element being driven by a second servomotor; and a control system electrically connected to the first servomotor and the second servomotor to independently control positioning of the first shoe element and the second shoe element relative to the workpart.
 20. A method for holding a workpart on a rotating chuck in a centerless grinding operation comprising the steps of:positioning the workpart on the chuck; rotating the chuck about a chuck axis such that a geometrical axis of the workpart is offset from the chuck axis; positioning a first shoe element in contact with the workpart with a first positioning servomotor; positioning a second shoe element in contact with the workpart with a second positioning servomotor; and holding the first shoe element and the second shoe element in a fixed position such that the workpart is maintained in the offset position relative to the chuck axis during a grinding operation. 